The present invention relates to an ignition timing control system for internal combustion engines which retards the ignition timing when knocking signals, generated in response to the detection of knocking by means of vibration or sound caused inside and outside the cylinders of an engine due to the pressure therein are detected.
The ignition timing of an engine must be determined in accordance with the engine conditions so as to ensure the optimum operation of the engine. Ignition timing control systems known in the art are generally so designed that the ignition timing is controlled in accordance with the conditions of an engine which are represented by the engine's speed, as detected by its centrifugal advance mechanism and the engine's intake vacuum, as detected by its vacuum advance mechanism.
It is known in the art that generally, to achieve optimum engine efficiency and fuel consumption, ignition should occur at a position near a so-called MBT (the Minimum advance for Best Torque) and consequently it is necessary to adjust the ignition timing to the MBT in accordance with the engine operating conditions.
However, if the ignition timing is advanced gradually under certain engine conditions, knocking will result and stable operation of the engine will be made impossible. Generally, the relation between the MBT and the knock initiating ignition timing is such that the knocking limit is reached before the MBT under low speed and low load operation. Also, the knocking limit tends to be influenced by atmospheric conditions, such as temperature, humidity, etc., and consequently the ignition timing control systems now in use are so programmed that the ignition timing is controlled in accordance with the engine parameters comprising the engine speed and the intake vacuum so as to be retarded considerably with respect to the MBT to prevent the occurrence of knock under all of the possible operating conditions of the engine. As a result, the output as well as the fuel consumption are not optimized.
The existance of a close correlation between ignition timing and cylinder pressure is well known in the art, that is, when a mixture is exploded in the cylinder, no harmonic components (frequency components which are usually on the order of 5 to 10 KHz) are superimposed on the cylinder pressure when there is no knocking, but such harmonic pressure variations are caused when knocking occurs.
The effect of such harmonic pressure variations is the generation of vibration or sound to the outside of the cylinder. Many different types of so-called knock feedback ignition systems have been investigated in which the ignition timing is controlled by detecting such vibration or sound.
A known system of this type is designed so that the presence of knocking is detected for every explosion stroke of each cylinder, whereby the ignition timing is controlled in response to each combustion phenomenon or detection signal in such a manner that the ignition timing is retarded by a predetermined angle upon occurrence of knocking and the ignition timing is advanced by a predetermined angle when it is determined that there is no knocking condition.
A great disadvantage of this type of system is that when the feedback is applied in a multi-cylinder engine, due to the difference in intake system for fuel distribution, mixture distribution, etc., among the cylinders as well as the difference in combustion chamber condition (deposit, compression ratio, etc.) among the cylinders, there occurs a great variation in combustion among the cylinders thus increasing the hunting range of the ignition timing and thereby causing surging and slugging.